Telephone system



SPL 2 1958 A. BACHER 2,850,572

TELEPHOW;v SYSTEM y @WV W ,Zw

SPf- 2 1958 A. BACHER 2,850,572

TELEPHONE SYSTEM Filed June 22, 1953 3 Sheets-Sheet 3 5)/ QWV /77 527,5/

United States Patent Oiliee Patented Sept. 2, lfa

TELEPHONE SYSTEM Adolf Bacher, Berlin-Charlottenburg, Germany, assigner to Siemens & Halske Aktiengesellschaft, Munich and Berlin, Germany, a German corporation Application June 22, 1953, Serial No. 363,235

7 Claims. (Cl. 179-15) This invention relates to signalling systems and is particularly concerned with a telephone system having circuit means for simultaneously transmitting a plurality of calls over a common transmission path by means of amplitude-modulated pulses. The common transmission path may extend over lines or may be wireless, employing a high frequency carrier. The pulses, serving as modulation carriers for the various calls or messages, are transmitted in staggered sequence. In known systems of this type, these staggered pulses are transmitted over the common transmission path, such as a trunk line, also during gaps in the transmission (for example a telephone conversation), during which gaps the pulses are of constant amplitude (unmodulated carrier amplitude). Therefore, interconnected subscribers while not talking, can listen in on conversations of interconnected subscribers talking over the same common transmission path, because of poor cross-talk damping, the latter being governed by the band width of the common path (pulse-end oscillation phenomena) and by the time lag between the pulses allotted to the various calls.

This is avoided in accordance with the present invention by providing, at the input end of the common path, means which are available for the individual calls to be transmitted; -from among the various staggered modulation carrier pulses (channels) fed to them, these means act upon those pulses only which are specific to the communication in question; and as soon as modulation starts on these pulses, said means impart to these pulses such mean values of amplitude and/or polarity as to enable other means, available for this call at the output end of the common path, to evaluate the modulation.

The circuit arrangement according to the invention also permits an increase in the number of calls that can be simultaneously transmitted over the common path, that is, in the number of channels. As this is possible only by shorteningthe time interval between the staggered pulses, it would normally necessitate an increase in the band width of the transmission path (increase in wattage and decrease in impedance), but this cannot be carried too far as it would involve excessive expense. The invention presents a different possibility; a called subscriber, for example, may be connected so as to receive communications audibly only if his allotted modulated pulse is received with a critical or adequate amplitude and/or with a predetermined polarity. To determine whether this is the case, there may be provided at the output end of the common path, for example, a discharge tube (biased if necessary) whose conductivity or ampliiication ratio is intermittently varied by the simultaneous application of the pulse voltage and of the modulated pulses received over the common transmission path, so that the tube can control the completion of the connection to the receiving set proper. The use of such an electronic gate makes it possible to move the pulses of the various communication channels closer to each other and thus to increase the number of channels without thereby reducing the cross-talk damping for listening channels.

The invention will now be explained in greater detail with reference to the accompanying drawings, in which Fig. l shows an example of the network scheme;

Figs. 2 and 2a illustrate embodiments of the modulation device;

Fig. 3 is an embodiment of the electronic gate; and

Fig. 4 shows a diagram to explain the operation.

Referring now to Fig. 1, the subscribers T111 to T114 can communicate with each other over a common line having a multiplex inlet ME and a multiplex outlet MA. Any speaking subscriber is connected to multiplex inlet ME while a listening subscriber is connected to multiplex outlet MA. To permit two-way conversation, each subscriber is connected to a branched link, one side of which is connected to ME while the other side is connected to MA.

Assume, for example, that subscriber T111 wants to make a call. Through a `branching link G1, to which a balance circuit N1 is connected, he rst reaches a pulse amplitude modulating device PAM1, the construction of which will be explained later. A pulse selector PZ1 allots to subscriber Tnl a definite pulse, for example P1, for the desired call; this pulse is supplied at a predetermined instant and periodically thereafter by a common pulse generator PG and is to be amplitude-modulated by the conversation; its mean value will then have a predetermined amplitude.

The pulse generator PG supplies, besides pulse Pl, additional pulses such as P2, P3, P4 in timed sequence but with the same periodicity, which serve as carriers in the transmission of other calls to other subscribers. This staggering or time displacement of the pulses provides for the individual communication channels. After having been allotted a pulse, the calling subscriber T111 determines the subscriber to be called, for example, T114, this being done in known manner which has no bearing on the present invention, for example, by dialing to operate a link selector device WE through control link Sti in such a way that this device WE makes pulse selector P24 available through selecting link K4. Pulse selector P24 allots pulse P1 to the called subscriber T114. The above described preparatory connection between calling and called subscribers T111 and T114 could, of course, be accomplished in any other known and suitable manner. ln accordance with the invention, the modulating device PAMl causes the allotted pulse P1 to be transmitted through the common line at a predetermined rhythm and with an amplitude having a critical mean value provided the pulse is modulated by the voice of subscriber T111. lt will be understood that instead of controlling the magnitude of the amplitude, it is possible to control the polarity of the pulse, or else both of these characteristics, as a function of the modulation in such a manner that audible communication is possible only while modulation actually takes place. Reference as to the magnitude of the amplitude is therefore intended to include either or both control factors.

The modulated pulse is transmitted over the common line also to the electronic gate ET4 which passes the pulse on to subscriber T114 only if the electronic gate ET4 has been opened in a manner presently to be described.

Conversations between subscribers T112 and T113 or between any other subscribers (not shown) are established over the common line or channel in a similar manner, employing pulses (such as P2, P3 or P4) which are displaced as to time relative to pulse P1 land which are modulated by the voice currents and then transmitted over the channel with the critical amplitudes. Again, these pulses reach the respective called subscribers only if their electronic gates are opened by the corresponding pulses having sufficient amplitude.

To explain this operation in greater detail, an embodiment of the modulating Vdevice PAMlis illustrated in Fig. 2, while Fig; 3 shows an embodiment Voan Yelectronic gate such as ET 4, both merely by way ,ofY example.Y In `Fig.,2 .the circuit portion surrounded by a dot-'and-.dash line, may be replaced by the circuit illustrated in Fig.f;2a. Y Referring rst to Fig. 2, the tube R01 in its normal condition has applied thereto, by way of resistorsfWlV and W12, a negative grid bias -Ug of such value that tube Rol is only partially open. Tube R03 is open, because its grid has a positive bias applied thereto, taken oi between resistors Wz'3 and Wz'4. Now when the calling subscriber T111 is connected Vto the device PAM1 of Fig. l, Vshown'in detail in Fig. 2, and when the pulse selector PZ1 has allotted to the call the pulse P1 from among the time negativepulse, such as Pla, will be applied to resistor W17. This pulse may be used to electengagement with and to condition Vfor connection, in known manner, a known and vsuitable exchange or switchboard device VE (Fig. 1) associated with the common channel.

' If the resistors Wz'5 and Wz'6 are of such values that Y WS receives a larger Ypulse than W6, this will modifyY `the grid potential of tube R02 so that the latter is opened;

a positive potential is thereby applied to `cathode resistor Wig, and this makes the grid of tube'R03 relatively negative so that a positive pulse Plb is applied to resistor Wi7.

This pulse may likewise VbeV Aused in known manner for Y any desired signalling or controlling operations.l

As already mentioned, dialing bythe calling subscriber T111 operates the link selector device WE over the control line St1 in such a manner that pulse selector PZ4 is conditioned over the selecting `line K4 to determine the desired subscriber Tiz4.Y Selector PZ4 connects itself, in a manner not shown, to'jthe output side of generator PG whichfsuppliespulse P1.4 Consequently, electronic gate ET4 also receives pulse P1 whichhas been alotted to the calling subscriber T111.Y AThe switching operations initiated thereby will be4 described presently. Y kOn the calling subscribers side, the'voice currents,rsupp1ied to the Y,device PAMl (FigfZ) by wayof inlet 1, are passed through repeater U, ampliier V, amplitude limiting de vice A and a rectiiier device' GA to furnish a D. C. potential which is applied to the grid of Vtube R01. This D. C; potential causes the tube to become fully conductive so thathigher voltage amplitudes are applied to resistors Wz'S and Wz'6 than was the case in the above illustrated instance of a mere signal transmission. The modulated voltage is further supplied to the grid of tube R02 by.way of resistor Wz'S, this grid also receiving the potential taken 0H fromiresistor W15. Thus, the current passing through tube R02 is Vproportional to the voice current amplitude occurring at resistor W1'5 while the pulse P1 lasts.

becomes more negative thanit was before. Accordingly', a positive pulse with modulation occurs at resistor Wi7;

and this pulse is transmitted by wayof the multiplex in let ME (Fig. l) of the common channel to switching deyice VE and then by way ofthe multiplex outletV MA to .the electronic gate ET4 of the called subscriber T114.V The potential taken otfibetween resistors W3 and W4 and applied to theV grid of tubeV R03, together with the positive-pulse taken ott betweengresistors WS and Wi6, de-

This rendersv the cathode of tube R03 Vmore positive than the grid, Vor in other words, the grid Y 4 Y Y termines the magnitude (mean value) of the modulation Vcarrier pulse P1, that is, it ixes its critical amplitude about which the modulation is symmetrically distributed. The pulses P1, modulated by the conversation, are transmitted by Way of multiplex inlet ME, switching deviceVE and multiplex outlet MA to the grid of the electronic gate ET4 (Fig. l) which is illustrated more in detail in Fig. 3.

VThe electronic gate ET4 (Fig. 3) is biased with a negative grid potential -Ug of such value that further, simultaneous, pulses must be applied to the grid to raise the tube to the working point of its operative range. The electronic gate ET4 is to be openedV in synchronism with the pulses P1; this opening is prepared, by the application of the negative pulse P1 to the cathode `by way of input link 2, to such an extent that the mean value (critical amplitude) of pulse'Pl, being simultaneously fed in by way of link 1, is capable of raising the electronic gate ET4 to its working point,whereby the component of the pulse P1 due to modulation can control the electronic gate ET4 in the operating range of the latter.

The operation just -described will be better understood upon consideration of Fig. 4, f Application vCif-pulse P1V by way of input link 2 shifts the grid bias from its'original value -Ug to point I, `and the simultaneous application of the pulse by wayV of link 1, yassuming its amplitude has the critical value, causes the tube to reach the working point AP, on both sides of whichV the tube current is controlled by the applied modulation M. It will ythus be seen that the pulse P1 with its component due to modulation, will be passed by the electronic gate ET4 only if the negative pulse P1 Yis applied by way of linlc2 with simultaneous application of the Vcritical amplitude f of this pulse Vby way-of link 1. Theanode resistor W9 will then receive pulsating voltages proportional to this` Y modulation component -of the pulse, and these voltages are transmitted by way of capacitorKol to a low-pass f2) may arise under certainV circumstances, depending on the selected magnitudes of resistors WS land W1'6.Y Such a negative pulse may be used to bias the grid of electronic gate ET4 (Fig. 3) still further. to the negative side whereby, in accordance with the tube characteristic of Fig. 4, the anode tail current is still further reduced and the cross-talk damping is thus enhanced. t.

The speech of subscriber *T111V is transmitted to subscriber T114, as it were through a definite channel, by the rhythmic transmission -of'pulsefPL The speech of the called subscriber T114 can be similarly transmitted to subscriber T111 by the use of the pulse-amplitude modulator PAM4 and electronic-gate ETI. Pulse -P1 again constitutes the allotted transmission fchannel.

Other subscribers engaged Vin calls have other pulses allotted to them,rwhich are .as to time displaced from Yeach other and fromY pulse P1 and which are similarly effective to :actuate corresponding pulse amplitude modulating devices and electronic gates..

Fig. 2a illustrates a modification of the circuit portion included within the dot-dash rectangle -of Fig. 2. In this embodiment, no D. C, voltage is derivedV from the voice 4currents fed in at 1, that is, from the modulation voltage, vand no suchtvoltage is-supplied to VVthe grid of the tube R01 (Fig. 2); instead, la Vspecial tube circuit including tubes R04 and-R05 is'provided which supplies the grid of tube R01 with the modulationpeaks practically The grid'biases ,are such that in 'assuma has bias O applied by way of resistor WilZ. This, in `conjunction with the negative bias -Ug provides, at point E, ya negative potential for the grid of tube R04 so that the latter is non-conductive. As soon Ias voice currents for modulation are received 'at 1, due to a subscribers talking, they are transmitted through amplier V, amplitude-limiting device A .and a differentiating element comprising capacitor K02 and resistor Wi13, whereby the positive peaks of the modulation are made eiective on the grid of tube R04 and render this tube conductive. This causes lowering of the plate potential at point B, whereby a negative pulse Iis applied by Way of capacitor K03 to point C which acts on the grid of tube R05 and makes the latter non-conductive. =Point D thus has a positive plate potential (O). The potential taken off from an intermediate point of resistor Wl applies, by way of resistor Wil (Fig. 2), such a grid potential to tube R01 that the latter becomes conductive. In addition, the feedback circuit from tube R to tube R04, comprising resistor WlS and capacitor K04 in parallel, applies to point E (the grid of tube R04) .a potential, even after disappearance of the positive modulation peak, ot such magnitude that tube R04 continues to remain conductive. This condition remains for a period of time governed by the time constant of capacitor K03 and resistor WiZ, namely until capacitor K03 has been discharged through resistor Wz'12, that is, until point C (the grid of tube R05) becomes more positive so that the tube R05 becomes conductive. This produces a neg-ative plate potential at D, which acts by way of resistor Wild and capacitor Kol-i upon the grid of tube R04 and makes the latter non-conductive. The above mentioned feedback results in rapid change-over of tube R05 from blocked to open, .and of tube R04 from open to blocked condition.

The next succeeding positive modulation peak causes the same operation as just described, so that tube R01 (Fig. 2) becomes conductive as soon as modulation takes place; as described with reference to Fig. 2, this causes the transmission of modulated pulses P1 by way of the common channel or trunk and through the electronic gate ET4 of the called subscriber, this gate opening in timing with the arriving pulses.

lt will be .apparent from the foregoing that the pulses such as P1, serving as `modulation carriers and as channels, initiate an eitective connection through the common trunk or channel only when modulation takes place; and that these pulses will reach the called subscriber (to complete the connection) only if his electronic gate is opened in timing with these pulses. During gaps in the conversation over a switched-through line, the channel-forming but unmodulated pulse passes through the line with insuiiicient amplitude. Consequently, even pulse-end oscillation phenomena in other connections during this period cannot result in cross-talk because the .pertinent electronic gates are not open during gaps in the conversation.

lf the control :of the electronic gates is to be made dependent on the polarity of the pulses, the grid bias -Ug must be so selected that the working point AP of the tube characteristic is reached only with the pulse polarity resulting from the occurrence of modulation.

Changes may be made within the scope `and spirit of the appended claims.

l claim:

l. in a telephone system having subscribers stations and having means forming a common channel for the simultaneous transmission by voice current modulated pulses of a plurality of calls extending between calling and called stations and having a generator for producing pulse series with the pulses of each series displaced as to time with respect to pulses of other of said series and having means for allotting a definite one of said pulse series to a call to be extended from a calling to a called station; apparatus for reducing the incidence of crosstalk, said apparatus comprising a iirst control de- 5 vice connected with the input side of said `common channel for receiving the definite series of Vpulses allotted to a call to be extended, the pulses of said allotted series to serve as voice current modulated carriers for the corresponding call, circuit means in said first control device responsive to voice currents received from the respective calling station for raising the mean value of pulses of the allotted pulse series as to amplitude thereof, a second control device connected with the output side of said common channel for receiving pulses .therefrom and for passing in the direction of the corresponding called station only the pulses of said allotted series the mean Iamplitude value of which had been raised by modulation with said voice currents, and means for demodulating said passed pulses and transmitting the resulting voice currents to the called station.

2. In a telephone system having subscribers stations and having means forming a common channel for the simultaneous transmision by voice current modulated pulses of a plurality of calls extending between calling and called stations and having a generator for producing pulse series with the pulses of each series displaced as to time with respect to pulses of other of said series and having mean for allotting a definite one of said pulse series to a call to be extended from a calling to a called station; apparatus for reducing the incidence of crosstalk, said apparatus comprising a first control device connected with the input side of said common channel for receiving the definite series of pulses allotted to a call to be extended, the pulses of said allotted series to serve as voice current modulated carriers for the corresponding call, circuit means in said first control device responsive to voice currents received from the respective calling station for raising the means value of pulses of the allotted pulse series as to amplitude thereof and for transmitting such voice current amplitude-modulated pulses to the input side of said common channel together with unmodulated pulses, a second control device connected with the output side of said common channel for receiving pulses therefrom and for passing in the direction of the corresponding called station only said voice current amplitude-modulated pulses, and means for demodulating said passed voice current amplitude-modulated pulses and transmitting the resulting voice currents to the called station.

3. A system and cooperation of parts according to claim 2, wherein said first control device comprises electron tubes, means for deriving a first Voltage from said carrier pulses, means for deriving a second voltage from said voice currents, means for combining said tirst and second voltages, and means for controlling said tubes by the potential of said combined voltages to effect transmission of said modulated pulses to the input side of said common channel.

4. A system and cooperation of parts according to claim 2, wherein said tir-st control device comprises electron tubes, means for deriving a rst voltage from said carrier pulses, means for deriving a second voltage from said voice currents and for rectifying said second voltage, means for combining said rst voltage with said second rectified voltage, and means for controlling said tubes by the potential of said combined voltages to eiect transmission of said modulated pulses to the input side of said common channel.

5. A system and cooperation of parts according to claim 2, wherein said lirst control device comprises a pair of serially connected electron tubes, means for deriving a iirst voltage from said carrier pulses, means for causing said voice currents to control said tubes so as to produce in the output of one of said tubes a second voltage, means for combining said iirst and second voltages, and further electron tube means controlled by said combined voltages to eiiect transmission of said modulated puises to the input side of said common channel.

6. A system and cooperation of parts according to ing a irst voltage from Ysaid carrier pulses, means for Vcausing said'voice currents to Vcontrol said tubes so as to produce in the output of one of said tubes a second voltage, feedback circuit means extending betweenv said serially connected tubes, said feedback means com' prising la capacitor and a resistor in paralleltherewith, the kfrequency response of said feedback means compensating the frequency response of a circuit component which comprises the grid-cathode-capacitance'of one of said tubes and a grid resistor connected in parallel therewith. Y

received yfromY the output side ofk said common channel adrfor passing `in the direction of thecalled stationfonly ,said"amplitude-modulated pulses. Y Y Y j I Referencesfited in the le ofrths Vpatent i' UNITED STATES PATENTS 2,543,736 Trevor Feb. 27, 1951 Houghton e Apr. 10,'19571 n 

